1. Field of the Invention
This invention relates to the design and construction of a piezoelectric speaker, particularly to the design and construction of a piezoelectric speaker with its piezoelectric transducer unit having an auxiliary diphragm for producing smooth sound and the horn member having a resonance device for increasing volume of sound output.
2. Prior art
A piezoelectric speaker is driven by a piezoelectric transducer which has an element being powered by a small disc of special piezo materials such as crystals, ceramics, towrmaline, Rochelle salt and so on, that changes its diameter when an electrical signal is applied across its surfaces. To convert this change in diameter to a sound output, the disc is bonded to a thin metal disc which acts as a restraining spring force on one surface of the former disc. An electrical signal of increasing amplitude causes the diameter of the former disc to change which causes the element to bend from a flat shape into a convex shape. When the polarity of the electrical signal reverses, the element will bend in the reverse direction into concave shape. If the applied electrical signal has a frequency of 2K Hz, then the metal disc will vibrate and produce sound at a frequency of 2K Hz.
A known piezoelectric speaker, as shown in FIG. 1, mainly comprises a tubular outer shell 10 provided with a tapered peripheral upper edge, a first cover 11 functioning as a horn and having a base 111 and an integrally formed circular side wall 112 provided with a peripheral edge which is divided into an inner flat edge 110 extending inwardly and an outer tapered edge engaged with the tapered upper edge of the shell 10. The first cover 11 is further provided with a plurality of air ports 12 in the angle portion and a hole 13 in the central portion of the base 111. A piezoelectric transducer element 20 comprises a copper disc 210 with an outer periphery corresponding to the inner side wall of the tubular shell 10 and a ceramic disc 211 of smaller diameter bonded to a central bottom side wall of the copper disc 210. A second cover 22 having a base and an integrally formed circular side wall is upwardly inserted into the cylindrical space of the shell 10 by means of a press fit so as to retain the transducer element 20 in a position between the first and the second covers 11, 22 by means of a peripheral edge thereof coacting with the flat edge 110 of the first cover 11.
The known piezoelectric speaker so constructed is found disadvantageous that, referring to FIG. 5, the output sound waves are tipped in a frequency range from 1.5K Hz to 4.0K Hz as said output sound waves are directly produced by vibration of the copper disc 210. The sound output is low in decibel as the sound waves lack the effect of resonance.
Another known piezoelectric speaker, as shown in FIG. 2, mainly comprises a pot-shaped shell 30 defining a chamber 301 having a stepped wall which provides a diaphragm seat 302 and a diaphragm 31 functioning as a supporter which includes a tapering outwardly extending circular side portion 311 having a first circular flat rim 312 angled inwardly and a circular crimped area 313 which terminates in a second flat rim 314 rested on and further secured to the diaphragm seat 302 with a ring 310 fastened to the diaphragm seat 302 by means of a press fit. A piezoelectric transducer element 32 comprises a copper disc 320 with a peripheral edge fastened to the first flat rim 312 by means of a heat seal or an adhesive and thus supported, and a ceramic disc 211 of smaller diameter bonded to a central portion of a bottom side wall of the copper disc.
It is found disadvantageous that the acoustic property and the volume in decibel are unsatisfactory as the sound waves are still directly produced by vibration of the copper disc and lack the effect of resonance.